It brings the total of confirmed exoplanets -- or "extra-solar
planets" -- to a staggering 825. However, the search for planets
in our own solar system has subsided since the
pioneering days at the end of the 18th century with the discovery
of Uranus and almost one hundred years later with the
identification of Neptune. The idea of another planet, dubbed
'Planet X,' inspired astronomers to keep searching for yet
another 100 years in a hunt that was full of twists and turns.

The hunt for Planet X began in 1781 when British astronomer Sir
William Herschel was studying stars in the constellation of
Taurus and noticed one star seemed slightly fuzzy or nebulous in
appearance. A few days later it seemed to have moved position --
he concluded it was a comet. Further study revealed it was
actually a planet -- Uranus -- the seventh planet in our solar
system and beyond the orbit of Saturn.

Detailed observations of Uranus' movement revealed an orbit that
seemed to be influenced by another, even more distant, object.
Mathematicians studying the data predicted the position of an
eighth planet before it was officially discovered. Visual
confirmation of Neptune's existence was announced in 1846.

Using the same techniques to study the orbital characteristics of
both Uranus and Neptune revealed they were both still being
tugged at by the gravitational force from another unknown object.
The search for the ninth planet in the solar system began and it
was American astronomer Percival Lowell who identified possible
candidates.

Some years after Lowell's death in 1930, Pluto was identified by
Clyde Tombaugh (an astronomer working at Lowell Observatory) and
was believed to be the final member of the solar system's
planetary family.

However, the 1978 discovery of Pluto's moon Charon reopened the
Planet X debate. Through accurate measurements of Charon's orbit,
the mass of Pluto could be deduced. Ultimately it showed that the
'ninth planet' couldn't possibly have affected the orbits of
Uranus and Neptune as observations appeared to show.

The renewed interest in Planet X was short lived as the Neptunian
flyby by Voyager 2 in 1989 revealed its mass was less than
thought. By reapplying this knowledge showed the outermost "ice
giant" planets were behaving exactly as they should and the
orbital perturbations were doewn to observational error. It seems
the myth of Planet X had finally died.

This could have been the end of the Planet X saga, but recent
studies of the Kuiper Belt -- a region of icy minor planets
located in the outermost reaches of the solar system -- suggest
this may not be the case.

It would be reasonable to expect the millions of frozen lumps of
rock would gradually decrease with distance from the sun, but at
a distance of 48 Astronomical Units (beyond the orbit of Pluto)
they seem to drop off suddenly, at the so called "Kuiper Cliff."

Maybe Planet X is responsible for this strange unexpected feature
in the outer edge of our solar system... or maybe not.

The Voyager and Pioneer spacecraft heading out of our solar
system haven't detected any substantial planets that might cause
the 'cliff,' but space is vast; the chances of a spacecraft
happening to fly past a previously undiscovered world would be
highly unlikely. Also, ground-based observatories and space
telescopes (like NASA's Wide-Field Infrared Survey Explorer) have
turned up little evidence.

But the jury is still out. Until an answer is found for the
Kuiper Cliff, the ghost of Planet X will remain as a tantalizing,
but unlikely, explanation.